Under a background of recent increases in communication traffic, the demand for optical communication transmission apparatuses is increasing. Introduction of optical communication transmission apparatuses has been vigorously carried out recently not only for optical repeating nodes introduced in backbone networks but also for local networks, and optical networks have been formed also for subscriber loops. Thus, the optical transmission system plays an important role with respect to world information networks.
In such an optical transmission system, a WDM optical amplifier repeater transmission system which can realize large-capacity and long-distance transmission while ensuring high reliability at a low cost, for example, by providing a WDM optical amplifier such as an erbium doped fiber amplifier (EDFA) for each repeater section (span) on the transmission line, becomes mainstream.
In the above-described WDM optical transmission system, under conditions such as where a transfer loss increases due to factors such as a long transmission line, the input level of the signal light to the optical amplifier decreases. Therefore, an optical signal-to-noise ratio (OSNR) indicating a ratio between signal light power and noise light power may deteriorate, to thereby deteriorate transmission characteristics of the WDM signal light. As one method of avoiding this, application of distributed Raman amplification (DRA) technology is effective, in which pump light is injected to the transmission line, to enable compensation of the transfer loss by utilizing amplification due to the effect of stimulated Raman scattering. In a system using distributed Raman amplification, since the level of the signal light that has propagated through the transmission line and been input to the optical amplifier such as the EDFA increases, the OSNR increases to improve the transmission characteristics, thereby increasing the number of repeater spans that can transmit the WDM signal light. Accordingly, the distributed Raman amplifier has already been put to practical use as an effective means for realizing long-distance transmission.
FIG. 9 is one example of a WDM optical transmission system that uses general distributed Raman amplification. Here a transmission section (Tx) 101 and a reception section (Rx) 102 are connected via a transmission line 103, and optical amplifiers 104 such as EDFAs are arranged at required intervals on the transmission line 103. Moreover Raman pump light sources (PumpLD) 105 are provided for injecting pump light into the transmission line 103 in each repeater section. According to such a system configuration, the WDM signal light is subjected to distributed Raman amplification on the transmission line 103 in each repeater section by the pump light from each Raman pump light source 105, to compensate for the loss, thereby improving the transmission characteristics of the WDM signal light that reaches the reception section 102.
Incidentally, in the distributed Raman amplification, it is known that a wavelength characteristic is generated in the output light level after amplification due to wavelength dependence of Raman gain. Regarding output wavelength characteristics in the distributed Raman amplification, in the WDM optical transmission system as illustrated in FIG. 9, level deviation between wavelengths is accumulated while being enlarged by the optical amplifier 104 (in-line amplifier) in each repeater section. Therefore, as schematically illustrated in the lower parts (A) to (C) of FIG. 9, level deviation between wavelengths in the reception section 102 may increase considerably, thereby causing a problem of deterioration of the transmission characteristics (for example, occurrence of a nonlinear phenomenon, OSNR deterioration, and exceeding an input range of a receiver).
In order to suppress deterioration of the transmission characteristics attributable to the gain wavelength characteristics of the distributed Raman amplification as described above, for example in Japanese Laid-open Patent Publication No. 2002-76482, as illustrated in FIG. 10, there is proposed a technique for compensating the wavelength characteristic of the WDM signal light after distributed Raman amplification by inserting an optical filter (GEQ) 106 having a fixed loss wavelength characteristic corresponding to the gain wavelength characteristic of the distributed Raman amplification, in each repeater section.
However, the output wavelength characteristics of WDM signal light after being subjected to distributed Raman amplification change according to system requirements such as the type of the transmission line, the number of wavelengths of the WDM signal light, and the setting of Raman gain. Therefore, a deviated portion between the fixed loss wavelength characteristics of each optical filter 106 and the output wavelength characteristics to be compensated by the respective optical filters 106 becomes a residual wavelength characteristic, and this accumulates through an optical amplifier repeater such as an in-line amplifier, causing a deterioration in the transmission characteristics, which is a problem.
Here the reason why the output wavelength characteristics change according to the system requirements will be explained briefly. In the transmission line, the loss wavelength characteristics are different because their compositions (materials) are different according to their type, and the output wavelength characteristics of the light that has been subjected to distributed Raman amplification on the transmission line serving as an amplification medium, also change according to the type of the transmission line. Moreover, when the WDM signal light enters into the transmission line, the signal light power on the long wavelength side increases due to the Raman effect, which is a physical phenomenon in which the power is transferred from the short wavelength side to the long wavelength side, and a tilt occurs in the output wavelength characteristics of the WDM signal light that has propagated through the transmission line. According to the above described Raman effect, the power shifting from the short wavelength side to the long wavelength side increases, as the number of wavelengths of the WDM signal light increases. Therefore, a large tilt occurs in the output wavelength characteristics of the transmission light. Moreover, since the efficiency of the Raman effect also changes because the effective core area is different according to the type of the transmission line, the size of the tilt due to the Raman effect also becomes different according to the type of the transmission line.
As a conventional technique that can reduce the aforementioned deterioration of the transmission characteristics due to the change of the system requirements, for example, in Japanese Laid-open Patent Publication No. 2001-7768 and Japanese Laid-open Patent Publication No. 2002-76482, a technique is proposed where the wavelength characteristics of the WDM signal light that has been subjected to distributed Raman amplification are monitored by using an optical spectrum analyzer or the like, and the power of the Raman amplification pump light having a plurality of wavelengths is adjusted so that the output wavelength characteristics decrease.
For example, in Japanese Laid-open Patent Publication No. 2006-189465 there is proposed a technique in which design value information relating to a pump light power ratio capable of canceling the residual wavelength characteristics which occur according to the system requirements, is stored in a database beforehand, and a design value having a pump light power ratio matched with the actual system requirements is extracted from the database and brought into a control operation.
However, in the above described conventional technique in which the output wavelength characteristics are monitored to adjust the pump light power, it is necessary to provide an expensive monitor device such as an optical spectrum analyzer, making the configuration complicated. Therefore there are problems such as an increase in apparatus cost, and problems with installation space. Moreover, since very complicated control is also required for adjusting the pump light power of each wavelength according to the monitored output wavelength characteristics, there is also a problem in practicability thereof.
Furthermore regarding the conventional technique in which the design value information of the pump light power ratio corresponding to the system requirements is stored in the database, a huge amount of data corresponding to various system requirements needs to be held in order to realize highly accurate control. For example, it is necessary to segmentalize transmission line loss (span loss) according to the required accuracy, and set the pump light power ratio corresponding to conditions such as the number of signal wavelengths and the Raman gain for each span loss, and then store this in the database. However, it is not easy to build up such a huge database, and even if it can be realized, a process for selecting a design value suitable for the actual system requirements from the database is required, thereby causing a problem in the control speed. That is to say, it is a problem how to specify a favorable pump light power ratio by using the requisite minimum database.